Trailers are subjected to such heavy loading on occasions that in most instances trailers are heavily constructed and are expensive. Most trailers are constructed to have a flat or nearly flat base which can be regarded as an horizontal base, surrounded by vertical walls, and the base and walls are reinforced to give mechanical strength which will withstand the various forces imposed. These forces include the dead weight forces of loading on the trailer floor, the live weight forces due to the "bounce" of such loading, the side thrust due to the use of trailers occasionally with a flowable load such as grain, gravel, sand or the like, and other dynamic forces which are encountered when the trailer negotiates deviations from direction in the vertical or horizontal plane. There are also dynamic inertia forces in a fore and aft direction.
It has long been recognized that certain plastics materials may be moulded advantageously to provide a low cost product, but heretofore mouldings have not been used extensively in trailers because of the mechanical forces which are imposed. It has been believed that mouldings are too weak to withstand these forces, unless the mouldings are made of material of such thickness as to be excessively expensive.